THERMODYNAMICS OF ADDITION OF H2, CO, N2, AND C-H BONDS TO M(PIPR3)2CL (M = IR, RH). AN UNPRECEDENTED METAL-CARBONYL BOND STRENGTH

The thermodynamics of interconversion of various complexes containing the unit IrL*2Cl (L* = P(iPr)3) have been investigated by calorimetry and equilibrium measurements. These complexes span a wide range of configurations including four- and five-coordinate d8 (IrL*2ClL‘, IrL*2Cl(CO)2) and five- and six-coordinate d6 (IrL*2ClRH and IrL*2ClRH(CO)). On the basis of kinetic experiments, a lower limit to the Ir−N2 bond dissociation enthalpy (BDE) of IrL*2Cl(N2) has been determined (36 kcal/mol). Using this value as an “anchor”, in conjunction with the relative addition enthalpies obtained calorimetrically, it is possible to derive lower limits for the absolute exothermicities of H2 (48 kcal/mol) and CO (72 kcal/mol) addition to IrL*2Cl; estimates can also be made for the addition of benzene and acetylene C−H bonds. These values are unusually high; indeed, the magnitude of the Ir−CO BDE is unprecedented. In addition, kinetic methods have been used to determine a lower limit of 29 kcal/mol to the Rh−N2 BDE of R...